I tried to compare two files and output customized string. Following is my script.
#!/bin/bash
./${1} > tmp
if ! diff -q tmp ans.txt &>/dev/null; then
>&2 echo "different"
else
>&2 echo "same"
fi
When I execute script, I get:
sh cmp.sh ans.txt
different
Files tmp and ans.txt differ
The weird part is when I type diff -q tmp ans.txt &>/dev/null. No output will show up.
How to fix it(I don't want line:"Files tmp and ans.txt differ")? Thanks!
Most probably the version of sh you are using doesn't understand the bash (deprecated/obsolete) extension &> that redirect both stdout and stderr at the same time. In posix shell the command &>/dev/null I think is parsed as { command & }; > /dev/null - it results in running the command in the background & and the > /dev/null part I think is ignored, as it just redirect output of a nonexistent command - it's valid syntax, but executes nothing. Because running the command in the background succeeds, the if always succeeds.
Prefer not to use &> - use >/dev/null 2>&1 instead. Use diff to pretty print the files comparison. Use cmp in batch scripts to compare files.
if cmp -s tmp ans.txt; then
&>, but also set -euo pipefail, previously recommended by some, e.g., Bash Strict Mode: redsymbol.net/articles/unofficial-bash-strict-mode . Not everyone may be aware of these changing recommendations.
cmpto compare files../${1} > tmpshould result in runningans.txtas a command - don't you get "permission denied" or other error on that line?sh- what is your implementation ofsh? Does the problem persist when running underbash?cmpis better thandiffwhen you only want to know if files are different, but don't care about generating a patch to transform one file to the other.cmp -sexists so you don't need to redirect stdout and stderr at all;if cmp -s file1 file2; then echo "Files are identical"; else echo "Files are different"; fi/bin/shand/bin/bashbehave differently even if they're both provided by bash (but it's on systems where/bin/shis provided byash,dash, etc. that the differences get really big). If you're using bash-only syntax, always use#!/bin/bashor#!/usr/bin/env bash, not#!/bin/sh.